The present invention relates to a method for forming thin film magnetic recording media containing magnetic particles with uniform barriers to magnetic reversal without significant sacrifice in signal-to-medium noise ratio (SMNR), and to magnetic media obtained thereby. The present invention is of particular significance or utility in the manufacture of thermally stable, high SMNR and hence high areal recording density magnetic recording media, suitable for use in computer-related applications, e.g., hard disks.
Magnetic recording media and devices incorporating same are widely employed in various applications, particularly in the computer industry for data/information storage and retrieval applications, typically in disk form. Conventional thin-film type magnetic media, wherein a fine-grained polycrystalline magnetic alloy layer serves as the active recording layer, are generally classified as xe2x80x9clongitudinalxe2x80x9d or xe2x80x9cperpendicularxe2x80x9d, depending upon the orientation of the magnetic domains of the grains of magnetic material.
A portion of a conventional longitudinal recording, thin-film, hard disk-type magnetic recording medium 1 commonly employed in computer-related applications is schematically illustrated in FIG. 1 in simplified cross-sectional view, and comprises a substantially rigid, non-magnetic metal substrate 10, typically of aluminum (Al) or an aluminum-based alloy, such as an aluminum-magnesium (Alxe2x80x94Mg) alloy or a suitable glass, ceramic, glass-ceramic, or polymeric material, or a composite or laminate of these materials, having sequentially deposited or otherwise formed on a surface 10A thereof a plating layer 11, such as of amorphous nickel-phosphorus (Nixe2x80x94P); a seed layer 12A of an amorphous or fine-grained material, e.g., a nickel-aluminum (Nixe2x80x94Al) or chromium-titanium (Crxe2x80x94Ti) alloy; a polycrystalline underlayer 12B, typically of Cr or a Cr-based alloy; a magnetic recording layer 13, e.g., of a cobalt (Co)-based alloy with one or more of platinum (Pt), Cr, boron (B), etc.; a protective overcoat layer 14, typically containing carbon (C), e.g., diamond-like carbon (xe2x80x9cDLCxe2x80x9d); and a lubricant topcoat layer 15, e.g., of a perfluoropolyether. Each of layers 11-14 may be deposited by suitable physical vapor deposition (xe2x80x9cPVDxe2x80x9d) techniques, such as sputtering, and layer 15 is typically deposited by dipping or spraying.
In operation of medium 1, the grains in the magnetic layer 13 are locally aligned by a write transducer, or write xe2x80x9cheadxe2x80x9d, to record and thereby store data/information therein. While moving over the surface of medium 1, the write transducer or head creates a highly concentrated magnetic field which alternates direction based on the bits of information to be stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the material of the recording medium layer 13, the magnetization direction of the grains (i.e., single magnetic domain particles) of the polycrystalline material at that location is aligned in the direction of the applied magnetic field. The grains retain their alignment after the magnetic field applied thereto by the write transducer is removed. Thus the magnetization direction of the grains matches the direction of the magnetic field applied thereto. The magnetization pattern of the recording medium layer 13 can subsequently produce an electrical response in a read transducer, or read xe2x80x9cheadxe2x80x9d, allowing the stored information to be read.
In perpendicular magnetic recording media, residual magnetization is formed in a direction perpendicular to the surface of the magnetic medium, typically a layer of a magnetic material on a suitable substrate. Very high linear recording densities are obtainable by utilizing a xe2x80x9csingle-polexe2x80x9d magnetic transducer or xe2x80x9cheadxe2x80x9d with such perpendicular magnetic media.
A typical perpendicular recording system 20 utilizing a vertically oriented magnetic medium 21 with a relatively thick soft magnetic underlayer, a relatively thin hard magnetic recording layer, and a single-pole head, is illustrated in FIG. 2, wherein reference numerals 22, 23, 24, and 25, respectively, indicate the substrate, soft magnetic underlayer, at least one non-magnetic interlayer, and vertically oriented, hard magnetic recording layer of perpendicular magnetic medium 21, and reference numerals 27 and 28, respectively, indicate the single and auxiliary poles of single-pole magnetic transducer head 26. Relatively thin interlayer 24 (also referred to as an xe2x80x9cintermediatexe2x80x9d layer), comprised of one or more layers of non-magnetic materials, serves to (1) prevent magnetic interaction between the soft underlayer 23 and the hard recording layer 25 and (2) promote desired microstructural and magnetic properties of the hard recording layer. As shown by the arrows in the figure indicating the path of the magnetic flux xcfx86, flux xcfx86 is seen as emanating from single pole 27 of single-pole magnetic transducer head 26, entering and passing through vertically oriented, hard magnetic recording layer 25 in the region above single pole 27, entering and travelling along soft magnetic underlayer 23 for a distance, and then exiting therefrom and passing through vertically oriented, hard magnetic recording layer 25 in the region above auxiliary pole 28 of single-pole magnetic transducer head 26. The direction of movement of perpendicular magnetic medium 21 past transducer head 26 is indicated in the figure by the arrow above medium 21.
With continued reference to FIG. 2, vertical lines 29 indicate grain boundaries of each polycrystalline (i.e., granular) layer of the layer stack constituting medium 21. As apparent from the figure, the width of the grains (as measured in a horizontal direction) of each of the polycrystalline layers constituting the layer stack of the medium is substantially the same, i.e., each overlying layer replicates the grain width of the underlying layer. Not shown in the figure, for illustrative simplicity, are a protective overcoat layer, such as of a diamond-like carbon (DLC) formed over hard magnetic layer 25, and a lubricant topcoat layer, such as of a perfluoropolyethylene material, formed over the protective overcoat layer. Substrate 22 is typically disk-shaped and comprised of a non-magnetic metal or alloy, e.g., Al or an Al-based alloy, such as Alxe2x80x94Mg having an Nixe2x80x94P plating layer on the deposition surface thereof, or substrate 22 is comprised of a suitable glass, ceramic, glass-ceramic, polymeric material, or a composite or laminate of these materials; underlayer 23 is typically comprised of an about 2,000 to about 4,000 xc3x85 thick layer of a soft magnetic material selected from the group consisting of Ni, NiFe (Permalloy), Co, CoZr, CoZrCr, CoZrNb, CoFe, Fe, FeN, FeSiAl, FeSiAlN, etc.; interlayer 24 typically comprises an up to about 100 xc3x85 thick layer of a non-magnetic material, such as TiCr; and hard magnetic layer 25 is typically comprised of an about 100 to about 250 xc3x85 thick layer of a Co-based alloy including one or more elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, and B, iron oxides, such as Fe3O4 and xcex4-Fe2O3, or a (CoX/Pd or Pt)n multilayer magnetic superlattice structure, where n is an integer from about 10 to about 25, each of the alternating, thin layers of Co-based magnetic alloy is from about 2 to about 3.5 xc3x85 thick, X is an element selected from the group consisting of Cr, Ta, B, Mo, and Pt, and each of the alternating thin, non-magnetic layers of Pd or Pt is about 1 xc3x85 thick. Each type of hard magnetic recording layer material has perpendicular anisotropy arising from magneto-crystalline anisotropy (1st type) and/or interfacial anisotropy (2nd type).
Efforts are continually being made with the aim of increasing the areal recording density, i.e., the bit density, or bits/unit area, by increasing the signal-to-medium noise ratio (hereinafter xe2x80x9cSMNRxe2x80x9d) of the magnetic media. However, severe difficulties are encountered when the bit density of longitudinal media is increased above about 20-50 Gb/in2 in order to form ultra-high recording density media, such as thermal instability, when the necessary reduction in grain size exceeds the superparamagnetic limit. Such thermal instability can, inter alia, cause undesirable decay of the output signal of hard disk drives, and in extreme instances, result in total data loss and collapse of the magnetic bits.
One proposed solution to the problem of thermal instability arising from the very small grain sizes associated with ultra-high recording density magnetic recording media, is by formation of a stacked layer structure wherein stabilization of a stacked pair of vertically separated ferromagnetic layers is provided via coupling of a ferromagnetic recording layer with another ferromagnetic layer or an anti-ferromagnetic layer, as has been recently proposed (see, e.g., E. N. Abarra et al., IEEE Conference on Magnetics, Toronto, April 2000 and U.S. Pat. No. 6,280,813 B1, the entire disclosure of which is incorporated herein by reference). According to this approach for providing stabilized magnetic recording media (hereinafter xe2x80x9cAFCxe2x80x9d media) comprised of at least a pair of the vertically spaced apart ferromagnetic layers are anti-ferromagnetically coupled together by means of an interposed thin, non-magnetic spacer layer. The coupling is presumed to increase the effective volume of each of the magnetic grains, thereby increasing their stability.
Still another proposed solution to the problem of thermal instability of very fine-grained magnetic recording media is to provide stabilization, hence increased SMNR, via formation of laminated media (hereinafter xe2x80x9cLMxe2x80x9d media), as for example, disclosed in U.S. Pat. No. 5,051,288, the entire disclosure of which is incorporated herein by reference. Such LM media comprise typically two or more stacked ferromagnetic layers separated by a non-magnetic spacer layer, where, in contrast to AFC media, the spacer layer generally is thicker and is provided for physically separating, rather than coupling, a pair of vertically stacked ferromagnetic layers; i.e., the strength of any magnetic coupling between the stacked ferromagnetic layers is smaller than the magnetic energies of the grains of each of the ferromagnetic layers.
Yet another approach for overcoming the problem of thermal instability of very fine-grained magnetic recording media is to form xe2x80x9chybridxe2x80x9d media, i.e., media comprised of a combination of two or more portions of different media types, e.g., longitudinal+perpendicular, AFC+laminated media, etc.
Regardless of the type of magnetic media, i.e., longitudinal, perpendicular, AFC, laminated, hybrid, etc., a major goal of magnetic thin film media design is formation of media wherein the magnetic particles (grains) thereof have uniform energy barriers to magnetization reversal. Design and fabrication of practical media with high or increased signal-to-media noise ratio (SMNR) must take into account three distinct distributions, i.e., the distribution of physical sizes of the individual magnetic grains, the switching field distribution, and the energy barrier distribution. The xe2x80x9cswitching fieldxe2x80x9d is the strength of the magnetic field required to be supplied by the transducer head to the medium in order to reverse the magnetic alignment of a single grain, and the xe2x80x9cenergy barrierxe2x80x9d is the resistance of the grain to thermally induced reversal of its magnetization direction. While the switching field and energy barrier are related to the size of the grain, they are not directly equivalent in practical media due to magnetic coupling (xe2x80x9cexchangexe2x80x9d) effects which cause the magnetic grains to consist of one or more physical grains. Thus, the number of physical grains is greater than the number of magnetic grains, and the mean effective size of a magnetic grain is greater than the mean physical size of the grains.
The SMNR performance of magnetic recording media is determined by the mean of the switching field distribution. A medium is more difficult to write when there is a significant xe2x80x9ctailxe2x80x9d to the higher switching fields. As a consequence, it is desired to eliminate such difficult-to-write switching fields. Long term stability of the magnetic grain alignment of written media is related to the energy barrier distribution, and is poor when there is a large number of grains with low energy barriers. Thus, it is desired to eliminate the presence of such low energy barriers. In practical media, there is some correspondence between the large energy barriers/large switching fields and the low energy barriers/low switching fields. Therefore, development of a practical, cost-effective process by which the energy barriers of the media can be altered such that they become more uniform, i.e., have a narrower distribution, is considered desirable.
According to current practices for design of magnetic media, the above-mentioned mean value of the energy barrier distribution is kept quite high in order to ensure that only an insignificant number of grains have energy barriers which are low enough such that they are subject to loss of alignment. If the energy barrier distribution were made narrower, however, the mean of the energy barrier distribution could be safely shifted to a lower value without incurring a penalty of increased signal decay. A typical approach for achieving a lower mean energy barrier distribution is to reduce the thickness of the magnetic layer(s) of the media, whereby an improvement in SMNR is obtained via a reduction in the effective write transducer-to-medium separation. However, such approach is limited by the necessity for maintaining a minimum thickness of the magnetic layer(s).
One method of obtaining media with uniform grain size, hence uniform energy barriers to magnetization reversal, is to deposit the ferromagnetic thin film recording layer(s) with as uniform a magnetic particle or grain size as possible, and to ensure that the uniformly sized magnetic particles or grains do not magnetically interact with neighboring magnetic particles or grains, e.g., as by forming non-magnetic grain boundaries. However, a significant impediment to obtainment of the desired uniformly sized magnetic particles or grains of the ferromagnetic thin film recording layers of the above described types of thin film magnetic recording media arises from the widespread use of sputtering techniques for depositing the ferromagnetic thin film recording layers. Specifically, ferromagnetic thin film layers prepared according to conventional sputtering techniques typically contain a distribution of magnetic particle or grain sizes, hence a distribution of energy barriers to magnetization reversal, and as a consequence, the stated goal of obtaining uniform energy barriers to magnetization reversal cannot be obtained via conventional sputtering techniques and methodology.
In view of the foregoing, there exists a clear need for improved, high areal recording density, thin film magnetic recording media (regardless of type) having ferromagnetic thin film recording layers wherein the presence of difficult-to-write switching fields is substantially eliminated or at least minimized, and wherein the distribution of energy barriers to magnetization reversal is narrowed, and methodology therefor which can be readily practiced in cost-effective manner (by use of conventional deposition techniques such as sputtering) at product throughput rates consistent with the requirements of automated manufacture of magnetic recording media, e.g., hard disks.
The present invention, therefore, provides methodology by which thin film magnetic recording media of various types and designs can be rapidly and costa, effectively manufactured (by means of conventional deposition techniques such as sputtering) with ferromagnetic thin film recording layers having magnetic grains with more uniform energy barriers to magnetization reversal, i.e., narrow energy barrier distributions of lower mean energy, but without a large number of grains with energy barriers low enough as to result in magnetization reversal.
An advantage of the present invention is an improved method of manufacturing a thin film magnetic recording medium.
Another advantage of the present invention is an improved method of manufacturing a thin film magnetic recording medium comprising magnetic particles with substantially uniform barriers to magnetization reversal.
Still another advantage of the present invention is an improved thin film magnetic recording medium.
Yet another advantage of the present invention is an improved thin film magnetic recording medium comprising at least one ferromagnetic thin film recording layer comprising magnetic particles with substantially uniform barriers to magnetization reversal.
Additional advantages and other aspects of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to an aspect of the present invention, the foregoing and other advantages are obtained in part by a method of manufacturing a thin film magnetic recording medium comprising at least one ferromagnetic thin film recording layer having magnetic particles with substantially uniform barriers to magnetization reversal, the method comprising steps of:
(a) providing a precursor structure for a thin film magnetic recording medium, the precursor structure including a surface and at least one ferromagnetic thin film recording layer having a first, higher coercivity, which first, higher coercivity may be greater than that which permits writing of the precursor structure, the at least one ferromagnetic thin film recording layer comprising magnetic particles having a distribution of energy barriers to magnetization reversal; and
(b) uniformly bombarding the entire surface of the precursor structure with particles of sufficient dosage and energy to:
(i) substantially equalize the energy barriers to magnetization reversal of the magnetic particles;
(ii) lower the coercivity of the at least one ferromagnetic thin film recording layer from the first, higher coercivity to a second, lower coercivity within a range of coercivities permitting writing of the bombarded at least one ferromagnetic thin film recording layer;
(iii) substantially retain the signal amplitude decay behavior of the precursor structure; and
(iv) limit the reduction in signal-to-medium noise ratio (SMNR) of the precursor structure to a pre-selected small amount.
According to an embodiment of the present invention:
step (a) comprises providing a precursor structure for a thin film magnetic recording medium having a first, higher coercivity from about 4,000 to about 5,500 Oe; and
step (b) comprises:
(i) substantially equalizing the energy barriers to magnetization reversal of the magnetic particles to within a range from about 50 to about 80 kT, depending upon the media design;
(ii) lowering the first, higher coercivity to a second, lower coercivity from about 2,500 to about 4,500 Oe;
(iii) limiting the change in the signal amplitude decay behavior of the precursor structure to not more than about 1%/decade; and
(iv) limiting the reduction in the SMNR of the precursor structure to not more than about 2 dB.
In accordance with various embodiments of the present invention:
step (a) comprises providing a precursor structure for a longitudinal, perpendicular, anti-ferromagnetically coupled (AFC), or hybrid thin film magnetic recording medium, wherein step (a) comprises providing the precursor structure by a process comprising forming the at least one ferromagnetic thin film recording layer having the first, higher coercivity by sputtering; step (b) comprises bombarding the surface of the precursor structure with neutral particles or ions, the neutral particles or ions having sufficient kinetic energy to dislodge atoms from the crystal lattice of the at least one ferromagnetic thin film recording layer and/or to result in implantation therein.
According to certain embodiments of the present invention, step (b) comprises bombarding the surface of the precursor structure with neutral particles or ions at a dosage sufficient to result in at least a pre-selected minimum reduction in coercivity; e.g., step (b) comprises bombarding the precursor structure with from about 10 to about 50 KeV ions selected from among helium, neon, argon, krypton, xenon, nitrogen, oxygen, and chromium ions.
In accordance with particular embodiments of the present invention, step (b) comprises bombarding the surface of the precursor structure provided in step (a) with 25 KeV Ar ions at a dosage, expressed as (ions/cm2)1/2/107, which is greater than about 2.5 to lower the first, higher coercivity of about 4,000-5,500 Oe to the second, lower coercivity of about 2,500-4,500 Oe.
According to still other embodiments of the present invention, step (b) comprises bombarding the surface of the precursor structure with neutral particles formed by electrically neutralizing ions prior to impact with the surface; e.g., step (b) comprises bombarding the precursor structure with neutral particles derived from ions selected from among helium, argon, neon, krypton, xenon, nitrogen, oxygen, and chromium ions.
Another aspect of the present invention is a thin film magnetic recording medium, comprising:
at least one ferromagnetic thin film recording layer comprising magnetic particles with substantially uniform barriers to magnetization reversal, the thin film magnetic recording medium made by a process comprising steps of:
(a) providing a precursor structure for a thin film magnetic recording medium, said precursor structure including a surface and at least one ferromagnetic thin film recording layer having a first, higher coercivity, the first, higher coercivity being greater than that which permits writing of the precursor structure, the at least one ferromagnetic thin film recording layer comprising magnetic particles having a distribution of energy barriers to magnetization reversal; and
(b) uniformly bombarding the entire surface of the precursor structure with particles of sufficient dosage and energy to:
(i) substantially equalize the energy barriers to magnetization reversal of the magnetic particles;
(ii) lower the coercivity of the at least one ferromagnetic thin film recording layer from said first, higher coercivity to a second, lower coercivity within a range of coercivities permitting writing of the bombarded at least one ferromagnetic thin film recording layer;
(iii) substantially retain the signal amplitude decay behavior of the precursor structure; and
(iv) limit the reduction in signal-to-medium noise ratio (SMNR) of the precursor structure to a pre-selected small amount.
According to embodiments of the present invention, the precursor structure for a thin film magnetic recording medium provided in step (a) has a first, higher coercivity from about 4,000 to about 5,500 Oe; and
the particle bombardment of step (b) comprises:
(i) substantially equalizing the energy barriers to magnetization reversal of the magnetic particles to within a range from about 50 to about 80 kT;
(ii) lowering the first, higher coercivity to a second, lower coercivity from about 2,500 to about 4,500 Oe;
(iii) limiting the change in the signal amplitude decay behavior of the precursor structure to not more than about 1%/decade; and
(iv) limiting the reduction in SMNR of the medium to not more than about 2 dB.
In accordance with various alternative embodiments of the present invention, the medium is a longitudinal medium, a perpendicular medium, an anti-ferromagnetically coupled (AFC) medium, or a hybrid medium; and in each instance the medium comprises a non-magnetic substrate with a thin film layer stack formed on at least one major surface of the substrate, the layer stack comprising the at least one bombarded ferromagnetic thin film recording layer.
Still another aspect of the present invention is a thin film magnetic recording medium, comprising:
(a) a non-magnetic substrate; and
(b) means on the substrate for providing at least one ferromagnetic thin film recording layer with magnetic particles with substantially equalized energy barriers to magnetization reversal.
Additional advantages and aspects of the present invention will become apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.